1. Field of the Invention
The present invention relates to an image forming apparatus, particularly to the image forming apparatus having a process, in which an unfixed image is transferred with toner to a sheet such as paper in transfer means and then the sheet is conveyed to fixing means to fix the unfixed image.
2. Description of the Related Art
Conventionally, in the image forming apparatus which forms an image with a developer including toner, a latent image is formed on a photosensitive member, the latent image is developed with the toner to transfer the developed image to a, sheet (recording material), and then the image is formed by heating and pressurizing the transferred image with a fixing device. Examples of the image forming apparatus which obtains a color image with the pieces of toner having plural colors include the image forming apparatus in which a color toner image primary-transferred to an intermediate transfer member in a superposing manner is collectively secondary-transferred to the sheet and the image forming apparatus in which each color toner image is sequentially transferred to the sheet in the superposing manner.
FIG. 8A to 8C is a view showing a transfer unit in which an intermediate transfer member is used and a neighborhood of the transfer unit. Referring to FIG. 8A, the toner image on a photosensitive drum 30 is primary-transferred in a superposing manner to an intermediate transfer belt (image bearing member) 35 which is an example of the intermediate transfer member. The color toner image on the intermediate transfer belt 35 is secondary-transferred in a collective manner to a sheet 100 by a transfer roller 39 which is an example of the transfer means. A predetermined bias voltage is applied between the transfer roller 39 and a transfer opposing roller 38 which is an example of a roller straining the intermediate transfer belt 35 by power supply means (not shown).
The sheet 100 to which the toner image is transferred is conveyed to a heating and fixing device 36 which is an example of the fixing means, and the image is fixed by applying heat and pressure with a fixing roller 44 and a pressure roller 45. A guide member 43 and a charge removal needle 42 are arranged between a transfer nip portion (pressure contact point of intermediate transfer belt 35 and transfer roller 39) and a fixing nip portion (pressure contact point of fixing roller 44 and pressure roller 45). The guide member 43 guides the sheet 100 to the fixing nip portion, and the charge removal needle 42 which is an example of an electrode member removes a charge on the charged sheet 100.
As shown in FIG. 8B, a front end portion of the sheet 100 conveyed in the transfer nip portion is guided to reach the fixing nip portion by the guide member 43. In order to achieve miniaturization of the image forming apparatus, a linear distance between the centers of the transfer nip portion and the fixing nip portion is set not more than 80 mm in a direction in which the sheet 100 is conveyed. Accordingly, as shown in FIG. 8C, the sheet 100 form a loop while sandwiched and conveyed by both the transfer nip portion and the fixing nip portion, and the transfer process, the charge removal process, and fixing process are simultaneously performed.
However, in the image forming apparatus shown in FIG. 8, a difference between a speed at which the sheet is conveyed in the transfer nip portion and a speed at which the sheet is conveyed in the fixing nip portion causes a change in loop state formed between the transfer means and the fixing means. The distance between the sheet and the electrode member is also changed as the loop state is changed, which causes a problem that an image defect is generated.